Your PocketPC has an internal mechanism intended to help user write text - word completeion feature. You enable or disable it and change some its options in System > Settings > Input.
But there is no way to add your word to dictionary or change words, that system learned from you (yes, it does it sometimes). Two dictionaries may exist: main and temporary, dyncompdict.
dat and dyncompdict. tmp. I can't tell right now, why there are two of them and whats each of them is intended for. Using this utility, you can create, edit and one of two dictionaries.
Dictionary will be alphabetically sorted, when saved. Words shorter than 4 letters, will be discarded - system will ignore such words anyway. Duplicate words also will be discarded.